Exhaust gas recirculation (EGR) has been used to lower emissions of internal combustion engines. Introducing EGR to engine cylinders can reduce engine pumping losses as well as formation of NOx. In WO2008/127755, a system is described having an EGR passage located downstream of a compressor on the intake side of an engine and upstream of a turbine on the exhaust side of the engine. The reference also describes a catalyst and particulate filter disposed in the exhaust system at a location downstream of the turbine. In one configuration, the reference describes flowing gases from a location downstream of the compressor at the intake side of the engine, to a location in the exhaust system upstream of the catalyst and particulate filter. Further, the reference describes adjusting the boost pressure when the pressure in the exhaust manifold is higher than the pressure in the intake manifold. The method may help to ensure flow from the intake manifold to the exhaust system, but the compressor may be operated such that the flow from the intake system to the exhaust system is low. Alternatively, the compressor may be operated such that a higher pressure than is desired such that engine fuel economy is reduced.
Recently, direct injection gasoline engines have been shown to improve engine performance and to reduce transient air-fuel disturbances that may be caused by fuel adhering to the intake manifold and ports of an engine. However, at higher engine speeds and higher engine loads, particulates may form in engine exhaust. Under some conditions, formation of the particulates may be related to the short amount of time between when fuel is injected to the cylinder and when combustion is initiated by a spark plug. Specifically, there may be only a short opportunity for the injected fuel to completely vaporize and form a homogeneous mixture before combustion is initiated. If a homogeneous air-fuel mixture is not formed in the cylinder before combustion is initiated, pockets of stratification may form, and soot may be produced by combusting rich areas within the cylinder air-fuel mixture. Particulate filters have been proposed as one way to reduce emissions of soot.
The inventors herein have developed a method for regenerating a particulate filter, comprising: operating a direct injection gasoline engine having intake and exhaust systems, at least one cylinder of said direct injection gasoline engine combusting a substantially stoichiometric air-fuel mixture; flowing gases from said intake system to said exhaust system at a location upstream of a particulate filter and downstream of a three-way catalyst; and adjusting boost pressure in response to at least a state of particulate filter regeneration.
By adjusting boost pressure in response to at least a state of particulate filter regeneration, boost pressure may be controlled such that there is sufficient flow between an intake system and an exhaust system without producing a shortage or excess of boost. In this way, a particulate filter may be regenerated with intake system gases without over or under boosting the engine. Further, the compressor boost pressure can be adjusted as the particulate filter regenerates to allow for changes in the rate of oxidation of material held by the filter so that matter held by the particulate filter is oxidized efficiently.
The present description may provide several advantages. Specifically, the approach may improve engine emissions by allowing a catalyst to operate in an efficient operating window while at the same time regenerating a particulate filter. Further, the present method allows boost to be adjusted such that under boost and over boost are reduced. Further still, the rate of particulate matter oxidation can be regulated by controlling the flow between the intake system and the exhaust system from feedback output from an oxygen sensor located downstream of the particulate filter and at least partially sensing oxidation of material held by the particulate filter.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.